1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to a cathode design having a first cathode active material of a relatively low energy density but of a relatively high rate capability and a second cathode active material having a relatively high energy density but of a relatively low rate capability. The first and second cathode active materials are contacted to their own current collectors. However, the cathode current collectors are all connected to a common terminal lead. A preferred form of the cell has the cathode terminal lead insulated from the casing serving as the negative terminal for the anode electrode. The present cathode design is useful for powering an implantable medical device requiring a high rate discharge application.
2. Prior Art
The capacity of an electrochemical cell is not only dependent on the electrode assembly design and packing efficiency, it also is dependent on the type of active materials used. For example, it is generally recognized that for lithium cells, silver vanadium oxide (SVO) and, in particular, ε-phase silver vanadium oxide (AgV2O5.5), is preferred as the cathode active material. This active material has a theoretical volumetric capacity of 1.37 Ah/ml. By comparison, the theoretical volumetric capacity of CFx material (x=1.1) is 2.42 Ah/ml, which is 1.77 times that of ε-phase silver vanadium oxide. For powering a cardiac defibrillator, SVO is preferred because it can deliver high current pulses or high energy within a short period of time. Although CFx, has higher volumetric capacity, it cannot be used in medical devices requiring a high rate discharge application due to its low to medium rate of discharge capability.
An attempt to use high capacity materials, such as CFx, by mixing it with a high rate cathode material, such as SVO, is reported in U.S. Pat. No. 5,180,642 to Weiss et al. However, electrochemical cells made from such cathode composites have lower rate capability. The benefit of increasing the cell theoretical capacity by using CFx, as part of the cathode mix is in part canceled by the lowering of its power capability in a high rate discharge application.
Another way to address the longevity issue is described in U.S. Pat. No. 5,614,331 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated hereby by reference. In this patent, a method of using a medium rate CFx cell to power the circuitry of an implantable defibrillator while simultaneously using a SVO cell to provide the power supply under high rate application for the device is described. The advantage of this method is that all of the high power SVO energy is reserved for the high power application such as charging a capacitor while the device monitoring function, for example monitoring the heart beat, which require generally low power requirements, is provided by the high capacity CFx system. This battery construction requires a very careful design to balance the capacities of the high power cell (SVO) and the low power cell (CFx) with both cells reaching end of service life at or near the same time. Such a balance, nevertheless, is very difficult to achieve due to the variable device usage requirements of a particular patient.